This invention relates to a method and device for the monitoring, in the comprehensive sense, of the whole of a superconducting system using as its principal component a superconducting functional element such as a superconducting electromagnet and incorporating a supporting structure therefor, i.e. for collective monitoring of the condition of the operation of the system, the condition under which the system is maintained in an extremely cooled state suitable for operation, and the condition under which the cooling of the system to the said extremely cooled state suitable for operation proceeds (hereinafter referred to as "precooling process"). It has heretofore been impossible to carry out comprehensive monitoring of a superconducting system in the manner described above because of the lack of an effective monitoring method capable of ensuring uniformized cooling for the system in the precooling process. The conventional art available for the monitoring of a system in operating condition has invariably relied solely upon a method involving the measurement of magnet voltage which has various disadvantages as will be described afterward. Thus, the monitoring of the operating condition obtained thereby has been quite unstable even during the most important phase of the operation.
As is universally known, a superconducting system is often used for the purpose of internally storing or conveying a high magnitude of energy and relies on a functional element such as, for example, a superconducting electromagnet or superconducting cable which is incorporated therein. If, during the operation of the system, a transfer from superconductivity to normal conduction (the S-N transition) takes place even partially in the superconducting functional element, the phenomenon of this transfer will, in the neighborhood of the critical point at which said transfer takes place, occur in the form of a positive feedback such as, for example, where heat generation excites further heat generation, no matter whether the said transfer may arise because of the influence of magnetic field, electric current or temperature. Once there is developed an unstable state, it tends to proceed frequently at a high velocity because of catastrophic quenching, a phenomenon in which the transfer of the state of superconductivity to that of normal conduction proceeds at a high rate of speed. Because of rapid growth of the unstable state coupled with the high magnitude of energy involved as mentioned above, the superconducting functional element undergoes an unexpected thermal load and a consequent breakage, with the possible result that the entire system may explode.
The recent trend has been toward gradual growth in the capacity of superconducting functional elements such as superconducting electromagnets and the total energy accumulated in the form of magnetic energy in these elements often reaches enormous magnitudes on the order of from megajoules to gigajoules. Thus, the destruction incurred by the functional elements by the S-N transition occurs in the form of an explosion so that anything in the surrounding is exposed to extreme danger. It is, therefore, highly desirable for the superconducting functional element in its operating condition to retain the superconducting state safely. For this reason, it is of very importance that whenever even a slight deviation from the superconducting state occurs in any part of the system or the functional element, it should be immediately detected in order to determine the existence of a dangerous state or the state immediately preceding the dangerous state. Generally the characteristic of the superconductor is such that its thermodynamically intrinsic critical values such as critical magnetic field, critical current and critical temperature can be determined statically based on the kind of material from which the superconductor is made. In various actually used superconducting devices, however, these critical values are vary greatly from one device or functional element to another by more practical and dynamic factors such as, for example, the difference in the manner of excitation in the case of a superconducting electromagnet, interaction with an adjacently located separate superconducting electromagnet, difference in structural sensitivity due to difference in physical structure and, in particular, difference in the manner and state of cooling.
In monitoring the superconducting devices of such nature, therefore, one cannot solely rely upon purely academic theories in the measurement of static critical values. At least in foretelling the danger described above, all the devices in the superconducting system must be kept under constant watch while they are in actual operation.
Heretofore, a method involving the measurement of voltage has been the only measure available for the detection of abnormal phenomenon during the operation of the system, which, if left to take its own course, will result in catastrophic quenching. This method effects the detection of such a phenomenon by amplifying a variation in the voltage which occurs when a transition from the state of superconductivity to that of normal conduction gives rise to a change in the resistance within the functional element. This method, however, has various defects such as are enumerated hereinafter by way of example and are encountered in the operation of a superconducting electromagnet serving as a functional element.
(i) If the sensitivity to voltage is enhanced for the purpose of clearly detecting at an initial stage the phenomenon likely to result in catastrophic quenching, it is quite likely that the measuring instrument will succeed in picking up only the minute dynamic variations in the magnetic flux generated in the superconducting electromagnet itself in the course of operation and, contrary to the intended purpose, only the background noise will be detected while any electric potential of a lower level than the noise will be hidden from measurement thereby. Besides, such an enhanced sensitivity to voltage tends to impair the stability of the measuring system itself. Thus, the enhancement of sensitivity is likely to prove harmful rather than helpful.
(ii) If a variable coil is electromagnetically connected to the superconducting electromagnet in use with a view to offsetting the electromotive force generated by inductance in the electromagnet so as to permit zero-point measurement indispensable for the measurement of voltage, the magnetic flux cannot always be expected to function exactly with geometric symmetry. In the case where a plurality of superconducting electromagnets are disposed within the entire system, there is a possibility that the zero-point adjustment to be effected on the electromagnets in use will be affected by the fluxes of other electromagnets located neareby and the desired cancellation of the electromotive force will not be obtained completely. In an extreme case, the unwanted effect of such adjacently located electromagnets could cause an error in the detection of voltage.
(iii) Direct detection of a loss of stability due to the occurrence of diamagnetic current or shielding current within the system is essentially impracticable because the voltage consequently produced cannot be detected at the terminal.
(iv) Although one can definitely tell that an abnormal state occurring somewhere in the system has developed into catastrophic quenching, he has no alternative but to rely on a number of detecting terminals disposed effectively in advance in the relevant electromagnet to ensure recognition of the precise position of the abnormal state, namely, to permit safe pinpointing of the abnormal state within the system. Such provision of detecting terminals makes the construction and actual fabrication of the electromagnet quite complicated.
(v) The method involving the measurement of voltage is such that even when a transition from the superconducting state to that of normal conduction is detected thereby, catastrophic quenching still cannot be prevented because the transition often proceeds too rapidly. The only measure that can be taken by an operator for repressing the rapid growth of the transition is to cut off the flow of electric current. In a superconducting system, this discontinuation of the current flow represents an instantaneous nullification of its function. Such an instantaneous stop of the system entails various problems.
In the light of the various disadvantages enumerated above, it is totally unsafe to rely solely upon any method involving the measurement of voltage such as has been adopted to date for the detection states preceding catastrophic quenching in the functional element within the superconducting system. The result of detection by such a method so much is affected by the various indefinite factors involved as described above that the early stages of the abnormal state which develops into catastrophic quenching cannot be detected.
Perfection of a satisfactory method for monitoring the superconducting system under its operating condition has been much desired. Generally in a superconducting system, in addition to the importance attached to the detection of early signs of catastrophic quenching in the functional element, the dimensional increase of the system as a whole has made it extremely important to ensure uniformization of the cooling of the component elements of the system, namely the functional element, its housing and those members which serve to support the functional element with reference to the housing and consequently permit required assembly of the superconducting system in the mechanical sense. Thus, it is necessary to adopt additionally a method for monitoring the system with respect to the condition of cooling, particularly for the purpose of detecting any loss of balance in the overall cooling of the system which tends to occur in the course of precooling, because the said loss of balance may possibly upset the equilibrium in shrinking thermal stress and consequent deformative strain in the component elements of the system, impart adverse effects upon the operation of the functional element and mechanically damage the container (occasionally giving rise to fracture through the development of cracks).
An object of the present invention is to provide a method and device for the monitoring of a superconducting system, which permits detection of incipient S-N transition, or even of states preceding the incipient state, occurring in the functional element of the superconducting system.
Another object of the present invention is to provide a method and device for the monitoring of a superconducting system, which enables a sign preceding the occurrence of an abnormal state in the entire superconducting system to be detected with high accuracy during the operation of the system or during the precooling of the system without being adversely affected by the magnetic field of the superconducting system being monitored.